This invention is directed to a reactivity control system for a light water breeder reactor (LWBR). In particular, the LWBR reactivity control comprises a stationary seed-blanket core arrangement comprising a radial arrangement of the fuel into a pattern of discrete seed regions that contain ThO.sub.2 -UO.sub.2 fuel pellets and discrete blanket regions that contain pure thorium dioxide fuel pellets. The United States Government has rights in this invention pursuant to Contract No. DE-AC12-76-SN00052 between the U.S. Department of Energy and the General Electric Company.
The continuous, world-wide growth of nuclear power based on current light water reactors will deplete readily obtainable supplies of the fissile fuel isotope uranium-235. While authoritative studies may disagree on the expected timing of this event, there is a general agreement that, to pursue nuclear power as a major energy source, the nuclear fuel cycle must take advantage of the potential energy available in the abundant fertile fuel isotopes, uranium-238 and/or thorium-232. Reactor technology in coming decades must shift away from the current light water reactor with a once-through fuel cycle toward more fuel efficient concepts, including fuel recycle, high energy converter reactors and breeder reactors.
Light water moderated converter reactors or breeder reactors using the thorium-232/uranium-233 fuel cycle are looked upon as attractive options for future nuclear reactors. The attractiveness of the thorium fuel cycle in a light water reactor derives from three major considerations; (1) the core, reactor equipment, primary system and balance of the plant are all based on the well-established technology of light water reactors; (2) fuel utilization is better than for the uranium/plutonium fuel cycle in a similar light water reactor application with recycled fuel and the thorium fuel cycle can achieve a self-sustaining breeder reactor system; (3) existing pressurized water reactor plants could be refitted with thorium fuel cycle converter cores, although the highest level of fuel utilizations are probably not achievable at full plant power ratings.
The atomic Energy Commission and its successor governmental agencies, ERDA and the Department of Energy, has attempted to demonstrate the potential of the thorium fuel cycle in light water moderated reactors from the mid-1970's. Various concepts are being explored, including pre-breeder, converter and advance breeder reactors including a scale-up of the Shippingport reactor operated by the Duquesne Light Company. A very recent development in this technology has advanced the concept that a practical commercial scale breeder may be made which does not rely upon a separate source, such as a pre-breeder and converter reactor to provide its initial load of fissile uranium-233. The concept includes a breeder reactor plant which becomes its own pre-breeder by using fuel elements of different dimensions for the initial pre-breeder core cycles.
The present invention is directed to a reactivity control system for the breeder concept of this type of pre-breeder/breeder reactor system. The reactivity control system for the breeder concept proposed in this application must perform all the functions of the reactivity control system in a commercial pressurized water reactor (PWR) but, in addition, it must perform those functions while minimizing the loss of neutrons to neutron poisons or other parasitic materials. For example, a typical present generation commercial PWR control system consists of soluble boron used in the primary coolant and poison control rod assemblies used for shutdown, regulating, and axial power shaping. This control system has the advantage of good axial and radial power distributions due to the uniform poisoning effect of soluble boron, but has the disadvantage of poor neutron economy due also to the presence of a relatively large amount of soluble boron. Accordingly, this type of control system is not conductive to a light water breeder reactor concept because of poor neutron economy present when one uses large quantities of boron.
The Shippingport light water breeder reactor control system consists of a movable fuel control. In this system, each active module of the core contains a central movable fuel assembly surrounded by a stationary blanket assembly. Reactivity control is accomplished by varying the axial position of the movable seed assemblies relative to the surrounding stationary blanket assembly. For a typical module, the movable speed volume is approximately 30% of the total active module volume. The Shippingport LWBR control system has the advantage of good neutron economy because it adjusts reactivity by using variable seed positions rather than a poison material. However, due to the movable fuel, the Shippingport LWBR system results in a higher axial power peaking not present in the commercial PWR. The reactivity control system of the present invention is able to achieve the neutron economy of the movable fuel Shippingport LWBR while maintaining the axial power peaking properties of commercial PWRs. Accordingly, the reactivity control system of the present invention is able to incorporate the advantages of the PWR control system and the LWBR control system of Shippingport without their attendant disadvantages.